Detection of a ground fault has been considered heretofore to be important as a safety countermeasure for a DC circuit such as a source circuit. If there is a ground fault point in the DC circuit, it will induce an electric shock accident upon contact with the insulation failure point of the DC circuit, and two ground fault points could cause a fire. One of methods for detection of such a ground fault is, as described in Patent Literature 1 below, a method of detecting a difference between amounts of direct currents flowing through a plurality of DC lines, through a detection core and thereby detecting a DC ground fault occurring in the measurement target lines. The other known methods include a method of connecting an AC power supply through a resistive element to a measurement point of a DC circuit and detecting the presence/absence of a ground fault by determining whether there is an in-phase component with an applied voltage in an electric current flowing through the resistor component, and a method of connecting a DC power supply and a resistive element between a measurement point of a DC circuit and the ground potential, applying a DC bias to the measurement point, and measuring a leak current through the resistive element, thereby detecting the present/absence of a ground fault.